Polyhydroxyalkanoates (PHAs) are one class of biodegradable polymers. The first identified member of the PHAs thermoplastics was polyhydroxybutyrate (PHB), the polymeric ester of D(-)-3-hydroxybutyrate. The biosynthetic pathway of PHB in the gram negative bacterium Alcaligenes eutrophus is depicted in FIG. 1. PHAs related to PHB differ in the structure of the pendant arm, R (FIG. 2). For example, R.dbd.CH.sub.3 in PHB, while R.dbd.CH.sub.2 CH.sub.3 in polyhydroxyvalerate, and R.dbd.(CH.sub.2).sub.4 CH.sub.3 in polyhydroxyoctanoate.
The genes responsible for PHB synthesis in A. eutrophus have been cloned and sequenced. (Peoples et al., J. Biol. Chem., 264, 15293 (1989); Peoples et al., J. Biol. Chem., 264, 15298 (1989)). Three enzymes: .beta.-ketothiolase (phbA), acetoacetyl-CoA reductase (phbB), and PHB synthase (phbC) are involved in the conversion of acetyl-CoA to PHB. The PHB synthase gene encodes a protein of M.sub.r =63,900 which is active when introduced into E. coli (Peoples et al., J. Biol. Chem., 264, 15298 (1989)).
Although PHB represents the archetypical form of a biodegradable thermoplastic, its physical properties preclude significant use of the homopolymer form. Pure PHB is highly crystalline and, thus, very brittle. However, unique physical properties resulting form the structural characteristics of the R groups in a PHA copolymer may result in a polymer with more desirable characteristics. These characteristics include altered crystallinity, UV weathering resistance, glass to rubber transition temperature (T.sub.g), melting temperature of the crystalline phase, rigidity and durability (Holmes et al., EPO 00052 459; Anderson et al., Microbiol. Rev., 54, 450 (1990)). Thus, these polyesters behave as thermoplastics, with melting temperatures of 50-180.degree. C., which can be processed by conventional extension and molding equipment.
Traditional strategies for producing random PHA copolymers involve feeding short- and long-chain fatty acid monomers to bacterial cultures. However, this technology is limited by the monomer units which can be incorporated into a polymer by the endogenous PHA synthase and the expense of manufacturing PHAs by existing fermentation methods (Haywood et al., FEMS Microbiol. Lett., 57, 1 (1989); Poi et al., Int. J. Biol. Macromol., 12, 106 (1990); Steinbuchel et al., In: Novel Biomaterials from Biological Sources. D. Byron (ed.), MacMillan, N.Y. (1991); Valentin et al., Appl. Microbiol. Biotechnical, 36, 507 (1992)).
The production of diverse hydroxyacylCoA monomers for homo- and co-polymeric PHAs also occurs in some bacteria through the reduction and condensation pathway of fatty acids. This pathway employs a fatty acid synthase (FAS) which condenses malonate and acetate. The resulting .beta.-keto group undergoes three processing steps, .beta.-keto reduction, dehydration, and enoyl reduction, to yield a fully saturated butyryl unit. However, this pathway provides only a limited array of PHA monomers which vary in alkyl chain length but not in the degree of alkyl group branching, saturation, or functionalization along the acyl chain.
The biosynthesis of polyketides, such as erythromycin, is mechanistically related to formation of long-chain fatty acids. However, polyketides, in contrast to FASs, retain ketone, hydroxyl, or olefinic functions and contain methyl or ethyl side groups interspersed along an acyl chain comparable in length to that of common fatty acids. This asymmetry in structure implies that the polyketide synthase (PKS), the enzyme system responsible for formation of these molecules, although mechanistically related to a FAS, results in an end product that is structurally very different than that of a long-chain fatty acid.
Because PHAs are biodegradable polymers that have the versatility to replace petrochemical-based thermoplastics, it is desirable that new, more economical methods be provided for the production of defined PHAs. Thus, what is needed are methods to produce recombinant PHA monomer synthases for the generation of PHA polymers.